1. Field of the Invention
The present invention relates to a power supply circuit for driving a liquid crystal display device.
2. Description of the Related Art
FIG. 1 shows the constitution of an example of a conventional power supply circuit for supplying electric power to, and driving, a passive matrix liquid crystal display device that is driven by a data drive circuit and a scan drive circuit. The power supply circuit of this example is operative to supply a data drive voltage to the data drive circuit. In FIG. 1, reference character 501 designates an input power supply. The value of a voltage supplied from this input power supply 501 is 6 V±1V or so.
Further, reference characters 502 and 504 denote resistors. Reference character 503, 505, 506, and 507 respectively designate a variable resistor, a diode group, a transistor, and a data drive voltage for driving a data drive circuit of the liquid crystal display device. Reference characters 508 and 509 denote capacitors. The resistor 502, the variable resistor 503, the resistor 504, and the diode group 505 are connected in series in this order. A terminal of the upper resistor 502 is connected to the input power supply 501. A cathode of the diode group 505 is connected to the ground.
The transistor 506 is an ordinary bipolar transistor. The collector, base, emitter of this transistor 506 are connected to the input power supply 501, a sliding terminal of the variable resistor 503, and a terminal of the capacitor 509, respectively. The other terminal of this capacitor 509 is connected to the ground. Furthermore, the capacitor 508 is connected between the base of the transistor 506 and the ground.
The data drive voltage 507 corresponds to a voltage for driving the liquid crystal display device. The upper resistor 502 is used for determining an upper limit of the data drive voltage 507, while the lower resistor 504 is used for determining a lower limit of the data drive voltage 507. Moreover, the variable resistor 503 is used for regulating a base current of the transistor 506.
The diode group 505 consists of two silicon diodes connected in series with each other and is provided for compensating for the temperature characteristic of the liquid crystal display device. That is, when a user changes a resistance value of the variable resistor 503, the data drive voltage 507 is regulated at a low current within a voltage range limited by the upper resistor 502 and the lower resistor 504. Thus, the brightness of the liquid crystal display device is controlled.
Furthermore, a scan drive voltage (not shown) outputted from the scan drive circuit for driving the liquid crystal display device is constant.
FIG. 2 is a graph illustrating brightness control ranges for controlling the characteristics of the liquid crystal display device, which include the brightness and the temperature characteristic thereof, in the case of using the conventional power supply circuit. FIG. 2 shows curves (namely, T-V curves) representing the dependence of the transmittance of the liquid crystal display device on the root mean square value of a voltage (level of a video signal) at certain temperatures in a normally white mode. In FIG. 2, reference characters 601, 602, 603, 604, 605, and 606 denote a high-temperature operating range, a low-temperature operating range, an automatic temperature correction range, a room-temperature operating range (indicated by a solid curve), a high-temperature T-V curve (indicated by a one-dot chain curve), and a low-temperature T-V curve (indicated by a two-dot chain curve), respectively. The room-temperature T-V curve 604 is a T-V curve obtained at a temperature of 20° C., and commences falling when the root mean square value of the voltage is about 1.9 (Vrms), and ceases falling when the root mean square value of the voltage is about 2.2 (Vrms).
The low-temperature T-V curve 606 is a T-V curve obtained at a temperature of 0° C., and commences falling when the root mean square value of the voltage is about 2.0 (Vrms), and ceases falling when the root mean square value of the voltage is about 2.3 (Vrms). The high-temperature T-V curve 605 is a T-V curve obtained at a temperature of 40° C., and commences falling when the root mean square value of the voltage is about 1.8 (Vrms), and ceases falling when the root mean square value of the voltage is about 2.1 (Vrms).
The low-temperature operating range 602 indicates a range for controlling the brightness of the liquid crystal display device at a temperature of 0° C. by using the power supply circuit shown in FIG. 1. Further, the high-temperature operating range 601 indicates a range for controlling the brightness of the liquid crystal display device at a temperature of 40° C. by using the power supply circuit shown in FIG. 1. The automatic temperature correction range 603 indicates a range for automatically correcting the brightness of the liquid crystal display device according to the temperature characteristic of the diode group 505 shown in FIG. 1.
Both of the low-temperature operating range 602 and the high-temperature operating range 601 are determined by the variable resistor 503 shown in FIG. 1. The difference between the low-temperature operating range 602 and the high-temperature operating range 601 depends upon the automatic temperature correction range 603.
In the case of the power supply circuit shown in FIG. 1, the operating ranges are increased by using the variable resistor 503, whose resistance is largely variable, so as to compensate for a range change caused according to the temperature characteristic of the liquid crystal display device. Thus, fine adjustment of the data drive voltage 507 cannot be achieved. Moreover, the range of the data drive voltage 507 changes with production variations in the input power supply.
Further, the division of the input power supply 501 by, for instance, resistance division using the upper resistor 502 and the lower resistor 504 reduces the significance of a voltage change caused by the temperature of the diode group 505 for temperature compensation.
Furthermore, an optimum value of the data drive voltage 507 is twice a voltage close to a threshold voltage (VthLCD) of the liquid crystal of the device, at which the optical characteristics thereof abruptly change. Therefore, it is inadvisable that a user controls the data drive voltage 507 by adjusting the variable resistor 503.